Move the tests to the subdirectory of each module.

[cosimoNigro]

Tests are accumulating in the same subdirectory, agnpy/tests/.
I prefer to create a tests subdirectory in each of the submodules, and put the tests there, on the line of what is done by Gammapy.

This way we could put - for example - all the tests related to the spectra module, in agnpy/spectra/tests and split the single file test_spectra.py that we have now, in several smaller files testing individual classes.

[cosimoNigro]

This has been fixed by #154.